working the tutorial, question on where @current_user is defined

Hi, very new to rails, working through the tutorial by Micheal Hartl, Im looking at module SessionsHelper after coding it per instructions, (see code below). in method def current_user=(user) I see @current_user getting assigned. However I have no idea where in the world its declaration is??? Can someone guide me here? thanks in advance...

Thanks!! I was wondering if I should learn Ruby first or go through the rails tutorial first, Micheal does have a short section on Ruby, but I didn't remember any discussion on dynamic attributes.. OK, that makes sense, I thought I tested it with a short pgm at the console, but must have had a different error... I just now tested it again and you're right. I also noticed that current_user is called each time a new page is rendered and assume that is something that rails does in parent methods.. Micheal indicates it is called, so it must be part of ActionController..